# Measurement of indirect CP asymmetries in $D^0 \to K^-K^+$ and $D^0 \to \pi^-\pi^+$ decays

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## Abstract

Time-dependent $CP$ asymmetries in the decay rates of the singly Cabibbo-suppressed decays $D^0\rightarrow K^-K^+$ and $D^0\rightarrow \pi^-\pi^+$ are measured in $pp$ collision data corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected by the LHCb experiment. The $D^0$ mesons are produced in semileptonic $b$-hadron decays, where the charge of the accompanying muon is used to determine the initial state as $D^0$ or $\bar{D}^0$. The asymmetries in effective lifetimes between $D^0$ and $\bar{D}^0$ decays, which are sensitive to indirect $CP$ violation, are determined to be \begin{align*} A_{\Gamma}(K^-K^+) = (-0.134 \pm 0.077 \; {}^{+0.026}_{-0.034})\% A_{\Gamma}(\pi^-\pi^+) = (-0.092\pm 0.145 \; {}^{+0.025}_{-0.033})\% \end{align*} where the first uncertainties are statistical and the second systematic. This result is in agreement with previous measurements and with the hypothesis of no indirect $CP$ violation in $D^0$ decays.

## Figures and captions

 Invariant mass distributions for (a) $D ^0 \rightarrow K ^- K ^+$ , (b) $D ^0 \rightarrow \pi ^- \pi ^+$ and (c) $D ^0 \rightarrow K ^- \pi ^+$ candidates. The results of the fits are overlaid. Underneath each plot the pull in each mass bin is shown, where the pull is defined as the difference between the data point and total fit, divided by the corresponding uncertainty. massal[..].pdf [21 KiB] HiDef png [238 KiB] Thumbnail [214 KiB] *.C file massal[..].pdf [21 KiB] HiDef png [246 KiB] Thumbnail [219 KiB] *.C file massal[..].pdf [21 KiB] HiDef png [207 KiB] Thumbnail [191 KiB] *.C file Raw $C P$ asymmetry as function of $D ^0$ decay time for (a) $D ^0 \rightarrow K ^- K ^+$ , (b) $D ^0 \rightarrow \pi ^- \pi ^+$ and (c) $D ^0 \rightarrow K ^- \pi ^+$ candidates. The results of the $\chi^2$ fits are shown as blue, solid lines with the $\pm1$ standard-deviation ($\sigma$) bands indicated by the dashed lines. The green, dashed lines indicate one $D ^0$ lifetime ($\tau=410.1\rm fs$). Underneath each plot the pull in each time bin is shown. Agamma[..].pdf [28 KiB] HiDef png [216 KiB] Thumbnail [181 KiB] *.C file Agamma[..].pdf [28 KiB] HiDef png [226 KiB] Thumbnail [190 KiB] *.C file Agamma[..].pdf [27 KiB] HiDef png [183 KiB] Thumbnail [164 KiB] *.C file Mistag probability, before subtracting the contribution from wrong-sign (WS) decays, determined with $D ^0 \rightarrow K ^- \pi ^+$ candidates. The result of the fit to the data points with an exponential function is overlaid (solid, blue line). The red, dashed line indicates the expected mistag contribution from WS decays. momega[..].pdf [21 KiB] HiDef png [208 KiB] Thumbnail [160 KiB] *.C file Measured values of $A_{\Gamma}$ for different magnet polarities and data-taking periods for (a) $D ^0 \rightarrow K ^- K ^+$ , (b) $D ^0 \rightarrow \pi ^- \pi ^+$ and (c) $D ^0 \rightarrow K ^- \pi ^+$ decays. The vertical line and error band indicate the average $A_{\Gamma}$ obtained from the combined data set. The error bars indicate the statistical uncertainty only. polari[..].pdf [13 KiB] HiDef png [112 KiB] Thumbnail [101 KiB] *.C file polari[..].pdf [13 KiB] HiDef png [110 KiB] Thumbnail [99 KiB] *.C file polari[..].pdf [13 KiB] HiDef png [106 KiB] Thumbnail [97 KiB] *.C file Animated gif made out of all figures. PAPER-2014-069.gif Thumbnail

## Tables and captions

 Contributions to the systematic uncertainty of $A_{\Gamma}( K ^- K ^+ )$ and $A_{\Gamma}(\pi ^- \pi ^+ )$ . The constant and multiplicative scale uncertainties are given separately. Table_1.pdf [47 KiB] HiDef png [99 KiB] Thumbnail [46 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [118 KiB] This ZIP file contains supplemetary material for the publication LHCb-PAPER-2014-053. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in variuous formats Fig1-S.pdf [14 KiB] HiDef png [208 KiB] Thumbnail [191 KiB] *C file

Created on 24 August 2019.Citation count from INSPIRE on 24 August 2019.